B737 FLC HAD PROPERLY DEFERRED YAW DAMPER AND MAINT FOUND A HYD LEAK.

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ARRIVED AT ACFT. DURING PREFLT; FOUND THERE TO BE AN MEL WITH THE YAW DAMPER INOP. I REVIEWED THE LOGBOOK AND FOUND THERE TO BE 2 WRITE-UPS CONCERNING THE YAW DAMPER. ONE DATED JUN/XA/00 AND ONE ON JUN/XB/00. AFTER THE SECOND WRITE-UP THE YAW DAMPER WAS PLACED ON MEL AND THE ACFT WAS RELEASED BY MAINT BACK INTO SVC. THE FO AND I REVIEWED OUR MEL BOOK AND FOUND THE ACFT COULD BE FLOWN WITHOUT A YAW DAMPER AND THERE WAS A SPECIAL PROC WHICH WE REVIEWED AND FOUND IT TO BE A MAINT FUNCTION. ON THE GND IN ORD; A MECH WENT INTO THE COCKPIT AND SHUT DOWN ALL ACFT PWR. WE ASKED WHAT WAS GOING ON AND HE STATED WE WERE LEAKING FLUID FROM UNDER THE APU. I THOUGHT HE MEANT WE HAD AN APU PROB. AFTER 10-15 MINS; HE RETURNED AND TOLD US WE HAD A HYD LEAK AND SEVERAL NUTS WERE HANGING BY THEIR SAFETY WIRE IN THE RUDDER. WE WENT WITH HIM TO INSPECT THE TAIL SECTION. WE WERE LIFTED VIA 'CHERRY PICKER' TO ON TOP OF THE ELEVATOR AND WERE SHOWN WHERE HYD FLUID HAD LEAKED AND EATEN THE PAINT AND A SEAL AWAY ON TOP OF THE ELEVATOR AND THROUGH 2 ACCESS DOORS WHERE THE NUTS WERE HANGING. NO WAY COULD THIS HAVE BEEN SEEN DURING ANY NORMAL PREFLT; WITH ITS LOCATION BEING ON TOP OF THE ELEVATOR. AT THIS POINT; MAINT GNDED THE ACFT.